Since their discovery in the early 1990s, a great deal has been learned about the composition and properties of carbon nanotube materials. This research has demonstrated that carbon nanotubes exhibit extraordinary mechanical, electronic and chemical properties, which has stimulated substantial interest in developing applied technologies exploiting these properties. Accordingly, substantial research is currently directed at developing techniques for organizing, arranging and incorporating carbon nanotube materials into useful functional devices.
Carbon nanotubes are allotropes of carbon comprising one or more cylindrically configured graphene sheets and are classified on the basis of structure as either single walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs). Typically having small diameters (≈1-30 nanometers) and large lengths (up to several microns), SWNTs and MWNTs commonly exhibit very large aspect ratios (i.e., length to diameter ratio≈103 to about 105). Carbon nanotubes exhibit either metallic or semiconductor electrical behavior, and the energy band structure of nanotube materials varies considerably depending on their precise molecular structure and diameter. Doped nanotubes having intercalants, such as potassium, have been prepared and the central cavities of nanotubes have been filled with a variety of materials, including crystalline oxide particles, metals, gases and biological materials.
Single walled carbon nanotubes (SWNTs), in particular, are identified as candidates for functional materials in a new generation of high performance passive and active nanotube based electronic devices. SWNTs are made up of a single, contiguous graphene sheet wrapped around and joined with itself to form a hollow, seamless tube having capped ends similar in structure to smaller fullerenes. SWNTs typically have very small diameters (≈1 nanometer) and are often present in curled, looped and bundled configurations. SWNTs are chemically versatile materials capable of functionalization of their exterior surfaces and encapsulation of materials within their hollow cores, such as gases and molten materials.
A number of unique properties of SWNTs make these materials particularly attractive for a variety of emerging applied technologies including, sensors, light emissive systems, flexible electronics, and novel composite materials. First, SWNTs are believed to have remarkable mechanical properties, such as tensile strengths at least 100 times that of steel or any known other known fiber. Second, the electron transport behavior in SWNTs is predicted to be essentially that of a quantum wire, and the electrical properties of SWNTs have been observed to vary upon charge transfer doping and intercalation, opening up an avenues for potentially tuning the electrical properties of nanotube materials. Finally, SWNTs have also be demonstrated to have very high intrinsic field affect mobilities (e.g., about 9000 cm2V−1 s−1) making them interesting for possible applications in nanoelectronics.
The astonishing electronic and mechanical properties of SWNTs, together with the ability to deposit nanotubes onto plastics and other unusual device substrates, make them well-suited for use in large-scale distributed electronics for steerable antenna arrays, flexible displays, and other systems. Recent work indicates that random networks of SWNTs can form effective semiconductor layers for thin-film transistor- (TFT-) type devices. The device mobilities that have been achieved with these networks are, however, still far below the intrinsic tube mobilities inferred from measurements of transistors that incorporate an individual tube (or small number of tubes) spanning the gap between the source and drain electrodes. The resistance at the many tube-tube contacts that are inherent in the networks may limit charge transport.
Large-scale, dense longitudinally aligned arrays of SWNTs may provide a means of avoiding these problems, thereby offering the possibility to exceed the device mobilities that can be achieved in the networks. Forming such arrays, patterning their coverage and, possibly, interfacing them with SWNT networks represent significant experimental challenges. Some degree of alignment can be obtained by casting SWNTs from solution, but dense arrays formed in this fashion usually involve large numbers of overlapping tubes. In addition, transistors that use solution-deposited tubes typically have properties that are inferior to those of transistors built with tubes grown directly on the device substrate by, for example, chemical vapor deposition (CVD). Arrays of SWNTs can be generated from random networks, formed by CVD growth or solution deposition, via orientation-selective ablation with linearly polarized laser pulses. This process has the advantage that it does not rely on chemistries or solvents that can alter the properties of the tubes; it is, however, an inherently destructive process. Electric-field assisted growth or fast heating can produce aligned arrays of SWNTs. However, high-density arrays that cover large areas have not been demonstrated with these techniques; device implementations have also not been described.
It will be appreciated from the foregoing that there is currently a need in the art for improved methods for generating arrays of longitudinally aligned carbon nanotubes useful for realizing passive and active nanotube electronic devices for a range of applications. Methods are needed that are capable of generating nanotube arrays on device substrates, including polymer and other flexible substrates, having specific, preselected nanotube orientations, positions and physical dimensions. Methods are needed that are capable of generating dense nanotube arrays capable of providing electronic properties, such as field affect mobilities, necessary for enabling high performance electronic devices.